Secondary beta deuterium isotope effects on acidity constants of ammonium ions are measured using a remarkably precise NMR titration method. Deuteration is found to increase the basicity of methylamine, dimethylamine, benzylamine, and N,N-dimethylaniline. The effect is attributed to a lowered zero-point energy of a CH bond adjacent to an amine nitrogen. The method permits a determination of the stereochemical dependence of the isotope effect in a locked piperidine, and it is found that deuteration is more effective when antiperiplanar to a lone pair. The values are consistent with a cos(2) dependence on dihedral angle, with no detectable angle-independent inductive effect.
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